Process for surface treatment of zinc-plated steel plates

ABSTRACT

A process of surface treatment of zinc-plated steel materials comprising electrochemically treating the material as an electrode in a basic electrolysis bath which is composed of an aqueous solution containing more than 0.01% at least one substance selected from the group consisting of aluminum biphosphate and magnesium biphosphate.

United States Patent Watanabe et al.

[ Aug. 5, 1975 PROCESS FOR SURFACE TREATMENT OF ZINC-PLATED STEEL PLATES [75] Inventors: Takashi Watanabe, Sagamihara-shi;

Eiichi Tarumi, Machida-shi; Akira Teramae, Yokohama-shi,, Japan [73] Assignee: Nippon Steel Corporation, Tokyo,

Japan 22 Filed: May 6,1974

21 Appl. No.: 467,577

[56] References Cited UNITED STATES PATENTS 3.330,744 7/1967 Wright et al. 204/56 R 3,335,074 8/1967 Wright et a1. -204/56 R Primary ExaminerR. L. Andrews Attorney, Agent, or FirmToren, McGeady and Stanger 5 7 ABSTRACT A process of surface treatment of zinc-plated steel materials comprising electrochemically treating the material as an electrode in a basic electrolysis bath which is composed of an aqueous solution containing more than 0.01% at least one substance selected from the group consisting of aluminum biphosphate and magnesium biphosphate.

5 Claims, No Drawings 1 PROCESS FOR SURFACE TREATMENT OF ZINC-PLATED STEEL PLATES- The present invention relates to a process for the surface treatment of zinc-plated steel plates.

Zinc-plating has been extensively applied for a long time for protecting steel materials from rust-formation and corrosion, and the methods thereof include electroplating and dipping in the molten metal and spraying themolten metal.

However, a coating film of zinc is'not free from the formation of white rust, which often results in a dirty appearance, consumption of the zinc film and unfavorable conditions for subsequent processing and coating. For this reason, zinc-plated steel materials have been further coated by the chromate treatment, to avoid white rust and improve the corrosion resistance. In the process of chromate treatment different baths and methods of treatment have so far been developed. But they are essentially similar to each other because they commonly employ chromic acid, dichromic acid or salts thereof. Such processes cost the least and produce excellent corrosion resistance so that chromate treated materials have recently been the most popular type of product. However, the most serious problem with this treatment is the way in which the solution and the material used in the treatment can be discarded without causing environmental pollution. Therefore a process for surface treatment, other than the chromate method, of zinc-plated steel materials is required.

It is obvious from the descriptions above that a process for the surface treatment of zinc-plated steel materials which does not include the chromate treatment, if it could be developed, will be provide benefits with respect to the environmental health problem.

The present inventors, through extensive investigations, have found that an electrochemical treatment of the materials in a particular electrolysis solution can remarkably improve the resistance of zinc-plated steel materials against white rust formation and as a result thereof problems with treating exhaust solutions can be solved all at once.

More particularly, they found that a zinc-plated steel material can be markedly prevented from forming white rust when the material is electrochemically treated in a bath which consists of an aqueous solution containing one or both of aluminum and magnesium biphosphates or the same solution as above and further containing one or more members selected from oxide sols, such as, sols of alumina, titanium oxide and magnesia, and hydroxide sols, such as, hydroxides of aluminum and magnesium. In this case, the concentration of aluminum biphosphate and magnesium biphosphate in the aqueous solution should be greater than 0.01% to assure sufficient corrosion resistance. The sols mentioned above should preferably be greater than The corrosion resistance of a zinc-plated steel material could be considerably improved by treating it by the process of this invention, and further features of the present invention are as follows.

The exhaust electrolytic solutions of the present invention can be treated with more ease than that re- 2 discarded after simply adjusting the pH value, and the phosphate ions can be easily removed. The process of this invention, which produces no environmental pollution, is a significant improvement when compared with conventional processes for the surface treatment of zinc-plated steel materials. In the present invention, electrochemical treatment may be carried out cathodically with either direct or alternating current or both superposed, but the cathodic treatment with a direct current is the best.

The process of this invention is also fundamentally different from conventional phosphate treatments in that the electrolysis performed in the basic electrolysis bath, containing water soluble biphosphates of aluminum and/or magnesium, produces a water insoluble high molecular weight substance on the surface of the zinc-plated steel material. Biphosphates of a metal other than Al and Mg can not produce a high molecular substance having rust preventing ability.

Zinc-plated steel materials to be treated by the present invention include zinc-coated alloy steel materials. Worked products of the latter materials can, of course, also be treated by the process of this invention. Further, a water soluble rust-preventing agent can be added to the electrolysis bath of the present invention, and the biphosphates can be combined with phosphate or condensed phosphate, but in the latter case, the pH value of the solution should be adjusted.

The present invention will be illustrated through examples and examples for comparison as follows.

EXAMPLE 1 Cold rolled steel sheets (0.8 mm thick) which were plated with zinc (40 g/m were used as test specimens. The specimens were treated in an electrolysis bath in which the concentration of aluminum biphosphate was varied and then submitted to the brine spraying test (JIS 2-2371). I

Table 1 shows the conditions of the treatment and results of the test. The bath was at room temperature, but theresults were the same even at an elevated temperature. The meaning of the marks is as follows:

@ No white rust observed 0': Less than 3% of white rust A 4 to 9% of white rust x 10 to 50% of white rust xx: More than 51% of white rust From Table 1 it is seen that the concentration greater than 0.01% of aluminum biphosphate is effective to treatment No treatment xx EXAMPLE 2 The same test specimens as appeared in Example 1 were treated in the electrolysis bath in which sols of alumina and titanium oxide were added to a solution of aluminum biphosphate. The results obtained are shown in Table 2.

Table 2 Result of brine spraying test for 24 hours Composition of Condition electrolysis bath 20 V 10 sec. Cathodic treatm e nt Test specimens were extruded with an Erichsen testing machine by 5 mm.

EXAMPLE 3 The test specimens were cold rolled steel sheets (0.8 mm thick) which were plated with zinc on the surface (40 g/m The specimens were treated in an electrolysis bath containing a solution of magnesium biphosphate of various concentrations and then submitted to the brine spraying test (.llS-Z-2871). Conditions of the treatment and results of the test are shown in Table 3. The bath was at the room temperature, but the same results could be obtained even at an elevated temperature. The meaning of the marks is as follows:

: None observed 0 Less than 8% of white rust A 4 to 9% of white rust x 10 to 50% of white rust xx: More than 51% of white rust It is seen from the results that an electrolysis bath containing more than 0.01% of magnesium biphosphate is excellent in capability to prevent white rust formation.

EXAMPLE 4 The same test specimens as those appeared in Example 1 were treated in an electrolysis bath containing a solution of magnesium biphosphate to which were added alumina sol, titanium oxide sol and magnesia sol. Results obtained are shown in Table 4. It is obvious Composition of from the tables that these additives are effective to improve corrosion resistance and more markedly the processability.

Table 3 Brine spraying test for 7 hours Condition of electrolysis bath electrolysis Current density on x cathode: DK= 2A/dm 10 sec.

magnesium biphosphate 0.005%

magnesium biphosphate 0.01%

ma esium biphosphate 0.1%

DK 8 A/dm 10 sec.

Table 4 Brine spraying test for 10 hours Condition of Composition of electrolysis electrolysis bath Cathodic electrolysis DK 2 A/dm 10 sec.

Magnesium biphosphate alumina sol 0.001%

Magnesium biphosphate alumina sol 0.005% Magnesium biphosphate alumina sol 0.01% Magnesium biphosphate 1% alumina sol 0.1% Magnesium biphosphate 1% magnesia sol 0.05% Magnesium biphosphate 1% titanium oxide sol 0.05% Magnesium biphosphate 0.5% magnesium hydroxide sol 0.01% Magnesium biphosphate 0.5% alumina sol 0.01% magnesia sol 0.01% Magnesium biphosphate 0.5% magnesia sol 0.001% Magnesium biphosphate 0.5% aluminum hydroxide sol 0.05% Magnesium biphosphate 0.5% aluminum biphosphate 0.5% alumina sol 0.005% I Magnesium biphosphate 0.5% aluminum biphosphate 0.5% alumina sol 0.01% 7 Magnesium biphosphate 0.5% aluminum biphosphate 0.5% aluminum hydroxide sol 0.01%

No treatment at all Test specimens were extruded with an Erichsen test machine by 5 mm.

As has been described in detail, the electrochemical treatment of zinc-plated steel materials in a bath containing a solution of aluminum biphosphate and/or magnesium biphosphate to which sol such as of alumina and magnesia has been added proved to he remarkably effective for preventing white rust formation. The material may be coated after the above treatment. In all these points, the proposed method is an excellent one which can replace the conventional chromate treatment.

What is claimed is:

1. A process for the surface treatment of zinc-plated steel material comprising electrochemically treating said material as an electrode in a basic electrolysis bath which is composed of an aqueous solution containing more than 0.01% of at least one substance selected from the group consisting of aluminum biphosphate and magnesium biphosphate.

2. The process according to claim 1, wherein one or with direct and alternating currents superimposed. 

1. A PROCESS FOR THE SURFACE TREATMENT OF ZINC-PLATED STEEL MATERIAL COMPRISING ELECTROCHEMICALLY TREATING SAID MATERIAL AS AN ELECTRODE IN A BASIC ELECTROLYSIS BATH WHICH IS COMPOSED OF AN AQUEOUS SOLUTION CONTAINING MORE THAN 0.01% OF AT LEAST ONE SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF ALUMINUM BIPHOSPHATE AND MAGNESIUM BIPHOSPHATE.
 2. The process according to claim 1, wherein one or more members selected from the group consisting of metal oxide sols and metal hydroxide sols are added to the basic electrolysis bath each in a concentration exceeding 0.005%.
 3. The process according to claim 2, wherein the metal oxide sols are alumina sol, magnesia sol and titanium oxide sol.
 4. The process according to claim 2, wherein the metal hydroxide sol is aluminum hydroxide sol and magnesium hydroxide sol.
 5. The process of claim 1, wherein the electrochemical treatment is a cathodic electrolysis with direct current or an electrolysis with an alternating current or with direct and alternating currents superimposed. 